This invention relates to the art of regulated power supplies for supplying RF drive to a load, such as a radio transmitter.
Whereas the invention will be described in conjunction with providing an RF drive for an AM transmitter, it is to be appreciated that the RF drive may also be utilized in an FM transmitter for use in radio broadcasting or an FM transmitter for the audio portion of a TV transmitter.
It is known in the art to provide an RF drive to the power amplifier stage in an AM transmitter. The RF drive typically receives an RF signal from an oscillator and amplifies the signal prior to employing it as a drive to a power amplifier. The RF drive includes a plurality of power amplifiers each supplied with an identical regulated DC voltage and each serving to provide amplification of the input RF signal. The amplified RF signals are then combined as an RF drive to be supplied to the power amplifier stage of the AM transmitter. Thus, the RF drive is the sum of the outputs of each of the amplifiers in the driver.
In the absence of drive regulation, the RF drive may vary somewhat with power line voltage resulting in variations of the regulated DC voltage supply, temperature variations taking place within the amplifiers in the driver, as well as variations in the performance of the RF oscillator. It has been known in the art to employ complex switching regulators or low efficiency linear regulators in an attempt to regulate the complete RF driver.
The U.S. Pat. No. 4,439,744 to M. Kumar et al., discloses a variable power amplifier employing a plurality of power amplifiers, each producing a different power level. Depending upon the total power level desired, one or more of the amplifiers is turned on to provide the desired output power. Each of the power amplifiers is either fully on or fully off and there is no sampling of the output RF drive to determine variations from a desired level for use in controlling a linear amplifier for supplying a DC voltage of a magnitude in accordance therewith to one of the power amplifiers for providing a regulated RF drive, as is accomplished in accordance with the present invention.